In recent years, production of semiconductor devices such as blue LEDs, white LEDs, and violet semiconductor lasers by using group 13 nitrides such as gallium nitride and application of such semiconductor devices to various electronic apparatuses have been actively studied. Existing gallium nitride semiconductor devices are mainly produced by vapor-phase methods: specifically, by heteroepitaxial growth of a gallium nitride thin film on a sapphire substrate or a silicon carbide substrate by a metal-organic vapor phase epitaxy method (MOVPE) or the like. In this case, since such a substrate and the gallium nitride thin film are considerably different from each other in terms of thermal expansion coefficient and lattice constant, dislocations (one type of lattice defects in crystals) are generated at a high density in the gallium nitride. Accordingly, it is difficult to provide gallium nitride of high quality having a low dislocation density by vapor-phase methods. Other than vapor-phase methods, liquid-phase methods have also been developed. A flux method is one of such liquid-phase methods and, in the case of gallium nitride, allows a decrease in the temperature required for gallium nitride crystal growth to about 800° C. and a decrease in the pressure required for gallium nitride crystal growth to several megapascals by using sodium metal as a flux. Specifically, nitrogen gas dissolves in a melt mixture of sodium metal and gallium metal and the melt mixture is supersaturated with gallium nitride and a crystal of gallium nitride grows. Compared with vapor-phase methods, dislocations are less likely to be generated in such a liquid-phase method and hence gallium nitride of high quality having a low dislocation density can be obtained.
Studies on such flux methods have also been actively performed. For example, since existing flux methods have problems that a gallium nitride crystal grows in the thickness direction (C-axis direction) at a low growth rate of about 10 μm/h and non-uniform generation of nuclei tends to occur at the gas-liquid interface, Patent Literature 1 discloses a method for producing gallium nitride that overcomes the problems. Specifically, by stirring a melt mixture of sodium metal and gallium metal, a flow is generated from the gas-liquid interface at which the melt mixture and nitrogen gas are in contact with each other to the inside of the melt mixture. Patent Literature 1 states that, as a result, the growth rate of a gallium nitride crystal increased to 50 μm/h or more and non-uniform generation of nuclei did not occur at the gas-liquid interface or on the internal wall surface of a crucible.